If I want to lose some weight by exercise and follow balanced healthy diet can I do it alone or should I consult doctor first?

You can go it alone. As long as you don't have any known concerning medical ailment or concerning symptoms like chest pain, dizziness on exertion, abnormally significant shortness of breath on exertion or significant joint/muscle pains. you also have to be incredibly motivated to seek out information about exercise/diet on your own or seek other professionals like a personal trainer or dietitian.
Depends. On your current BMI. If you are just overweight and highly motivated you could easily do it. However people who has a lot of weight to loss and need motivation have better chances with the help of a Bariatrician.
Can do alone if... Presuming you are in good general health it should not be necessary to consult a doctor first.The key to losing weight is not a temporary diet but permanent changes. Sugars & simple carbs from grains esp. cause weight gain. It's also wise to avoid fried foods. I like Dr. Furhman's diet: http://tinyurl.com/ktampgg Dr. Weil has useful info: http://tinyurl.com/m8avwzh See his "discuss that" link too.